Transportation of highly flammable fluids requires a high level of caution. One particular problem with which the present invention is concerned is the prevention of static electric discharges near the flammable substance. Differences in voltage potential can exist between different pieces of equipment used in the filling and transportation of these highly flammable substances. For instance, a voltage potential difference may exist between the transport vehicle and the container from which the vehicle is loaded or the equipment used in the filling or discharging processes. For purposes of this discussion, the terms loading rack and filling station are used interchangably and refer to the locations where tank trucks are loaded with flammable fluids. If there is a difference in voltage potential between the vehicle and the loading rack (or filling station), such as might be caused by static electricity, a static electric discharge will occur when a metal part of the vehicle comes in contact with a metal part of the filling station. This is most likely to occur when the nozzle of the hose from the filling station makes contact with the filling nozzle of the vehicle or some similar situation. Static electric discharge at this moment is extremely hazardous since this might ignite the flammable substance and cause an explosion.
The present invention is intended for use at loading racks where Class I, Division 1, groups A, B, C or D hazardous substances, as defined by the National Electrical Code, Article 500, are loaded into tank trucks for transportation. Most gasoline and petroleum products such as diesel fuel, jet fuel and automobile fuel fall into one of these catagories, as do gases such as acetylene and hydrogen.
Many filling stations are therefore equipped with a grounding system in order to prevent static electric discharges of this type. The simplest systems provide a cable which is electrically connected to the filling station at one end and which has a clamp at the other end that can be connected or clamped to a metal part of the vehicle when in the filling station. If there is a voltage potential difference between the filling station and the vehicle, a static electric discharge will occur at the time and place where the cable contact is made and the filling station and vehicle will thereafter remain at the same ground. Even though a static electric discharge still occurs when using the grounding cable, the point of connection to the vehicle can be made at a location remote from the filling nozzle or any other area where the flammable substance may be, thereby eliminating any risk of igniting the flammable substance.
Unfortunately, however, it has been found that the workers at the loading racks or filling stations sometimes neglect to connect the cable to the vehicle. Improvements have been made to the simple grounding cable by adding circuitry that will prevent any filling operations from occurring by cutting off the electricity to or otherwise disabling the pumping systems and other equipment unless a proper ground contact has been established. One method of accomplishing this task is to provide on the clamp which is to be connected to the vehicle, an ohmeter circuit that measures the resistance between the jaws of the clamp. The clamp is provided with insulation material such that there is no metal contact between the two jaws of the clamp when it is fully closed and not attached to anything. The ohmeter circuit is connected to a disabling circuit which disables the filling equipment if the resistance between the jaws exceeds a certain level and enables the equipment when it is below the specified level. The resistance between the jaws of the clamp when it is not connected to metal is theoretically infinite, while the resistance when the clamp is connected to a piece of metal and the jaws both contact the metal is theoretically zero. Therefore, when the clamp is connected to the vehicle, the resistance between the jaws will be below the threshold and the filling equipment will be enabled. Unfortunately, this grounding system can be easily defeated by connecting the clamp to any piece of metal whether it is on the vehicle or not.
Many of the vehicles used for transporting flammable fluids are equipped with overfill protection devices which also must be connected to the loading rack and are intended to disable the pumping equipment when the limit of the vehicles cargo capacity is reached. Overfill protection circuits typically comprise two separate units. The first unit is mounted on the vehicle and comprises sensors within the cargo tank which detect the level of the fluid within the cargo tank and related circuitry for providing the information from the sensors to the second unit. The second unit is mounted on the loading rack and comprises circuitry for disabling the pumping equipment when the sensors indicate that the vehicle is fully loaded. The use of overfill protection circuitry on the vehicle requires a second electrical cable to be connected between the vehicle and the loading rack in order to couple the sensor circuitry to the base unit circuitry. As in the case of the grounding systems of the prior art, the workers at the loading racks frequently neglect to connect the overfill protection circuitry to the loading rack, thereby defeating this safety feature also.
Therefore, it is an object of the present invention to provide an improved self-proving grounding system.
It is a further object of the present invention to provide a self-proving grounding system that will enable the filling equipment if, and only if, the filling equipment and the vehicle are electrically grounded.
It is another object of the present invention to provide a self proving grounding system that can be used in conjunction with an overfill protection system to provide a single cable connection between the loading rack and the vehicle to complete both the overfill protection circuit and provide the self-proving ground.
It is yet another object of the present invention to provide a self proving grounding system which is easy to operate yet not easily defeated.